Image forming apparatus with a charging amount acquisition unit that performs a charging amount acquisition operation for forming a measurement toner image on an image carrier

Abstract

An image forming apparatus includes a storage unit and a charging amount acquisition unit. The charging amount acquisition unit performs a charging amount acquisition operation for forming a measurement toner image on the image carrier while changing the frequency of the alternating current voltage of the development bias with a potential difference in a direct current voltage between the developing roller and the image carrier being kept constant, acquiring a tilt of a measurement straight line representing a relationship between the change amount of the frequency and a density change amount of the measurement toner image, and acquiring the charging amount of the toner based on the acquired tilt of the measurement straight line and the reference information in the storage unit.

The invention claimed is: 
     
       1. An image forming apparatus comprising:
 an image carrier that is rotated and carries a toner image obtained by developing an electrostatic latent image which is formed on a surface of the image carrier; 
 a charging device that charges the image carrier to a predetermined charging potential; 
 an exposing device that exposes the surface of the image carrier charged to the charging potential, based on predetermined image information so as to form the electrostatic latent image, the exposing device being disposed in a rotational direction of the image carrier downstream with respect to the charging device; 
 a developing device that includes a developing roller that is rotated, carries developer including toner and carrier on a peripheral surface of the developing roller, and supplies the toner to the image carrier so as to form the toner image, the developing device being disposed in a predetermined development nip portion in the rotational direction downstream with respect to the exposing device so as to oppose the image carrier; 
 a transfer unit that transfers the toner image carried on the image carrier to a sheet; 
 a development bias applying unit that applies a development bias obtained by superimposing an alternating current voltage on a direct current voltage to the developing roller; 
 a density detecting unit that detects density of the toner image; 
 a storage unit that stores reference information in advance for each toner charging amount, the reference information relating to a tilt of a reference straight line representing a relationship between a change amount of a frequency of the alternating current voltage of the development bias and a density change amount of the toner image in a case where the frequency is changed with a potential difference in the direct current voltage between the developing roller and the image carrier being kept constant; and 
 a charging amount acquisition unit that performs a charging amount acquisition operation for forming a measurement toner image on the image carrier while changing the frequency of the alternating current voltage of the development bias with the potential difference in the direct current voltage between the developing roller and the image carrier being kept constant, acquiring a tilt of a measurement straight line representing a relationship between the change amount of the frequency and a density change amount of the measurement toner image based on the change amount of the frequency and a result of detecting density of the measurement toner image in the density detecting unit, and acquiring a changing amount of the toner included in the measurement toner image formed on the image carrier based on the acquired tilt of the measurement straight line and the reference information in the storage unit, 
 wherein 
 the storage unit stores three or more frequencies in the alternating current voltage of the development bias in advance, the three or more frequencies being referred to by the charging amount acquisition unit in the charging amount acquisition operation, 
 the charging amount acquisition unit forms the measurement toner images for one and the other of a maximum frequency and a minimum frequency of the three or more frequencies, forms the measurement toner image for a frequency between the maximum frequency and the minimum frequency, and acquires the tilt of the measurement straight line based on the result of detecting density of the formed three or more measurement toner images, and 
 the reference information stored in the storage unit is set such that as the toner charging amount becomes smaller the tilt of the reference straight line becomes greater, and wherein the reference information stored in the storage unit has a first toner charging amount where the tilt of the reference straight line is negative and also has a second toner charging amount where the tilt of the reference straight line is positive. 
 
     
     
       2. The image forming apparatus according to  claim 1 , wherein the charging amount acquisition unit acquires the measurement straight line based on the result of detecting density of each measurement toner image according to a method of least squares every time when the measurement toner image is formed for the frequency between the maximum frequency and the minimum frequency, determines the tilt of the measurement straight line to be acquired when a determination coefficient in the method of least squares satisfies a predetermined condition, and acquires a charging amount of toner included in the measurement toner image formed on the image carrier based on the acquired tilt of the measurement straight line and the reference information in the storage unit. 
     
     
       3. The image forming apparatus according to  claim 2 , wherein the predetermined condition is that a relation of R2≥0.9 is satisfied when the determination coefficient is represented by R2. 
     
     
       4. An image forming apparatus, comprising:
 an image carrier that is rotated and carries a toner image obtained by developing an electrostatic latent image which is formed on a surface of the image carrier; 
 a charging device that charges the image carrier to a predetermined charging potential; 
 an exposing device that exposes the surface of the image carrier charged to the charging potential, based on predetermined image information so as to form the electrostatic latent image, the exposing device being disposed in a rotational direction of the image carrier downstream with respect to the charging device; 
 a developing device that includes a developing roller that is rotated, carries developer including toner and carrier on a peripheral surface of the developing roller, and supplies the toner to the image carrier so as to form the toner image, the developing device being disposed in a predetermined development nip portion in the rotational direction downstream with respect to the exposing device so as to oppose the image carrier; 
 a transfer unit that transfers the toner image carried on the image carrier to a sheet; 
 a development bias applying unit that applies a development bias obtained by superimposing an alternating current voltage on a direct current voltage to the developing roller; 
 a density detecting unit that detects density of the toner image; 
 a storage unit that stores reference information in advance for each toner charging amount, the reference information relating to a tilt of a reference straight line representing a relationship between a change amount of a frequency of the alternating current voltage of the development bias and a density change amount of the toner image in a case where the frequency is changed with a potential difference in the direct current voltage between the developing roller and the image carrier being kept constant; and 
 a charging amount acquisition unit that performs a charging amount acquisition operation for forming a measurement toner image on the image carrier while changing the frequency of the alternating current voltage of the development bias with the potential difference in the direct current voltage between the developing roller and the image carrier being kept constant, acquiring a tilt of a measurement straight line representing a relationship between the change amount of the frequency and a density change amount of the measurement toner image based on the change amount of the frequency and a result of detecting density of the measurement toner image in the density detecting unit, and acquiring a changing amount of the toner included in the measurement toner image formed on the image carrier based on the acquired tilt of the measurement straight line and the reference information in the storage unit, 
 
       wherein:
 the storage unit stores three or more frequencies in the alternating current voltage of the development bias in advance, the three or more frequencies being referred to by the charging amount acquisition unit in the charging amount acquisition operation, 
 the charging amount acquisition unit forms the measurement toner images for one and the other of a maximum frequency and a minimum frequency of the three or more frequencies, forms the measurement toner image for a frequency between the maximum frequency and the minimum frequency, and acquires the tilt of the measurement straight line based on the result of detecting density of the formed three or more measurement toner images, and 
 the charging amount acquisition unit performs a first charging amount acquisition operation at a first peak-to-peak voltage of the alternating current voltage of the development bias, performs a second charging amount acquisition operation at a second peak-to-peak voltage, which is higher than the first peak-to-peak voltage, of the alternating current voltage of the development bias, and performs a charging amount distribution acquisition operation for acquiring distribution of the toner charging amount based on results in the first charging amount acquisition operation and the second charging amount acquisition operation. 
 
     
     
       5. The image forming apparatus according to  claim 4 , wherein the charging amount acquisition unit acquires the measurement straight line based on the result of detecting density of each measurement toner image according to a method of least squares every time when the measurement toner image is formed for the frequency between the maximum frequency and the minimum frequency, determines the tilt of the measurement straight line to be acquired when a determination coefficient in the method of least squares satisfies a predetermined condition, and acquires a charging amount of toner included in the measurement toner image formed on the image carrier based on the acquired tilt of the measurement straight line and the reference information in the storage unit. 
     
     
       6. The image forming apparatus according to  claim 5 , wherein the predetermined condition is that a relation of R2≥0.9 is satisfied when the determination coefficient is represented by R2.